The present invention refers to a high-speed turbo pump, and in particular to a turbo molecular pump or a circulation fan.
During operation of fast rotating shafts, a situation may occur in which the natural bending frequency equals or approximates the rotary frequency of the rotor. In such cases, undesired bending vibrations of the rotor shaft occur that could result in the destruction of the machine. Also when running up the rotor shaft to its operating number of rotations, the natural bending frequency often has to be passed. With mechanically supported systems having an over-mounted rotor, the transition from the rotor-side bearing to the rotor frequently is a spot of very limited flexural strength.
Generally, the natural bending frequency of a rotor shaft is reached only with turbo machines rotating very fast. The term xe2x80x9chigh-speed turbo machinexe2x80x9d refers to a machine with a number of rotations above about 27,000 rpm.
Turbo molecular pumps are known in which the rotor shaft is supported in cantilevered fashion in steel ball bearings or hybrid ball bearings (ceramic ball bearings). At the lower end, the rotor shaft has a portion of large diameter to which magnets are fastened which are part of an electric motor driving the rotor shaft. The upper shaft section on which the rotor is mounted, has a lesser diameter and it is supported by a ball bearing at the transition to the thicker shaft section.
It is an object of the present invention to provide a high-speed turbo machine with increased security against undesirable resonant vibrations.
According to the invention, the object is solved with the features of claim 1. Accordingly, the bearing of the rotor shaft is improved over the prior system in that a ball bearing is used whose end face supporting the rotor is enlarged.
This enlargement is achieved by enlarging the first end face of the inner race of the ball bearing towards the outside. Thus, at this location, the inner bending moments of the rotor shaft can be transmitted with lesser deformation. The flexural strength increases and, thus, the bending frequency also increases. The bending at rotary frequencies below the natural bending frequency decreases. By increasing the flexural strength, the natural bending frequency is increased so that it can no longer be reached by the operating frequency. Thus, undesirable resonant vibrations are avoided.
The inner diameter of the ball bearing is fixedly determined by the shaft diameter of the rotor shaft. According to the invention, the outer diameter of the inner ring is increased, whereby the supporting end face of the inner race is enlarged. This end face has an outer diameter that is at least as large as the reference diameter of the ball bearing minus 34% of the ball diameter. In this manner, a relatively large end face is obtained at which the supporting part of the rotor is supported. The end face of the supporting part should be at least as large as the first end face of the inner race of the ball bearing cooperating therewith, so as to obtain a support over a large surface.
One advantage of the present invention is that it increases the flexural strength of the rotor shaft.
Another advantage resides in increasing the natural bending frequency of the bearing shaft above its operating frequency.
Another advantage resides in avoiding resonances.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following brief description of the preferred embodiments.